Solar cells are electrical devices capable of converting solar energy into electrical energy without the generation of pollutants or other undesirable byproducts. As a result, solar cell technology has become of great interest in recent years as part of the worldwide push toward environmentally friendly alternatives to petrochemical-based energy production.
A solar cell commonly comprises a semiconductor that absorbs sunlight. Although sunlight includes light having wavelengths within the range of approximately 300 nanometers (nm) to 1200 nm, most semiconductor-based solar cells only absorb light within the range of approximately 300 nm to approximately 800 nm. The absorbed light gives its energy to electrons within the semiconductor, which excites them into the conduction band of the semiconductor. When a voltage is applied to the semiconductor, the flow of excited electrons is enabled, which gives rise to an electric current. This current can then be used to power an external device or charge an electrical storage device, such as a battery. Unfortunately, solar cells tend to be inefficient energy converters and quite expensive to manufacture. As a result, the cost to generate electrical energy using conventional solar cells has not been competitive with fossil-fuel-based energy generation, which has limited the adoption of solar-cell technology.
Recently, thin-film solar cells have been introduced, wherein the solar cells that are a few microns thick are formed on relatively inexpensive, large substrates (e.g., glass, plastic, stainless steel, etc.), rather than the more traditional relatively expensive, smaller silicon substrates. This advance offers the potential for cost reductions as well as an opportunity to employ more exotic materials to improve energy-conversion efficiency.
One type of thin-film solar cell that has received widespread attention is the dye-sensitized solar cell (DSC). DSCs have been demonstrated with relatively high power-conversion efficiency and low cost. Further, they offer promise for improving light harvesting in the 600-900 nanometer (nm) wavelength sub-range of solar light, which could increase power-conversion efficiency over 15%.
A classic DSC structure comprises a transparent anode deposited on the back of a transparent substrate, such as glass, on which a thin layer of titanium dioxide (TiO2) is formed. This substrate is then immersed in a mixture of a photosensitive dye and solvent. After soaking the film in the dye solution, a thin layer of dye remains covalently bonded to the surface of the titanium dioxide. This substrate is then joined with another substrate that comprises a thin layer of electrolyte disposed on a conductive electrode. When joined, the two substrates substantially seal in the electrolyte to keep it from leaking.
In a DSC, electron-hole free-carrier pairs are generated when light hits the photosensitive dye. The free electrons quickly diffuse across the titanium dioxide film and reach the anode before they can recombine with free holes. The dye molecules, therefore, are oxidized (i.e., become positively charged) due to their loss of an electron to the titanium dioxide. They recover their neutral state, however, by stealing electrons from iodide ions in the electrolyte, which thereby oxidize to form positively charged iodine. The iodine then diffuses to the opposite electrode (i.e., the cathode). As a result, the electrons and the holes become separated so that these charges can be collected at the different electrodes where they can form an electric current for an external circuit. The electrolyte enables rapid reduction of the dye molecules thus impeding recombination of the electron hole pairs, referred to as “charge splitting.” If the electrodes are connected through an external circuit, current will flow through the solar cell, enabling the electrolyte to regain its initial state.
Unfortunately, DSCs comprising liquid electrolytes have several drawbacks, including leakage of the electrolyte, and electrolyte volatility. Further, liquid electrolytes are typically corrosive, making them difficult to work with in a fabrication environment.
Solid-state DSCs (ss-DSCs) provide an alternative to liquid-based DSCs and overcome many of the issues faced by liquid-based DSCs, however. In an ss-DSC, the liquid electrolyte is replaced by a solid-state hole-transport material. The most common device architecture includes a porous layer of titanium dioxide nanoparticles whose surface area is coated with a photosensitive dye, and which is impregnated with a hole-transport material. The porous layer of titanium dioxide nanoparticles acts as a “scaffold” that can hold large numbers of the dye molecules in a three-dimensional matrix, enabling the inclusion of many more dye molecules on the surfaces of its internal pores than could be included just on the outside surface area of a solar cell. In addition to avoiding the problems of electrolyte leakage, volatility, and corrosiveness, solid-state hole transport material also offers a potential for higher energy-conversion efficiency.
Conventional solid-state DSCs are not without problems, however. First, electron-hole recombination in the devices leads to reduced energy-conversion efficiency. Second, the hole-transport material tends to fill the pores of the porous titanium dioxide layer incompletely when applied. These factors combine to limit the practical thickness for ss-DSCs to only about two microns. This thickness is significantly less than the thickness desired for efficient light absorption in the active layer. The resultant poor light absorption further reduces the energy-conversion efficiency of ss-DSCs.
In order to improve light absorption in the active layer, efforts have been directed toward the development of dyes that more strongly absorb light, as well as toward improved titanium dioxide nanostructure layers that have more internal surface area available for dye adsorption. Such efforts have had limited success in improving the overall energy-conversion efficiency of ss-DSCs, however.
An ss-DSC having improved energy-conversion efficiency as well as improved cost competitiveness with respect to other energy conversion methods would represent a significant advance in the state of the art.